JEFFERSON CITY, Mo. (AP) - More than 1 million people are expected to participate in what amounts to the largest-ever public opinion poll on the nation's new health care law.

Missouri on Tuesday will become the first state to the test the popularity of President Barack Obama's top policy accomplishment with a statewide ballot proposal attempting to reject its core mandate that most Americans have health insurance.

The legal effect of Missouri's measure is questionable, because federal laws generally supersede those in states. But its expected passage could send an ominous political message to Democrats seeking to hang on to their congressional majority in this year's midterm elections.